Our Department has "on call" volunteers.  Potential volunteers interview with me and may not be able to commit to a 4 or 5 hour shift one time weekly, but have all the other qualities I look for in a volunteer.  These individuals I offer a "on call" postion.  I cross train them in two areas of their interest and our need.  When their is a need (i.e. sick, vacation, etc.) I begin phoning until one is available. My department has had "on call" volunteers for about 4 years and it has worked out GREAT!